The rather low values of \(T_0\)  should be noted (see Tables REF ,REF ). Especially for the samples with a high carbon content, they are near the lower limit of the temperature range (9–40 K) where the temperature dependence of conductivity follows Eq. (REF ). This fact can indicate a large carrier localization length \(\xi \) . In a large number of cases for systems with the VRH conduction mechanism, \(\xi \)  can be estimated using magnetoresistance measurements, since, according to VRH theory [1]}, [2]}, the magnetoresistance is positive (because of the field-induced compression of the wavefunction of localized carriers) and (in not a very strong magnetic fields)
\(\ln [\rho (B)/\rho (0)]\propto (\xi /L_B)^4(T_0/T)^{3/(d+1)}\) 
